Of the recording media, an optical disk removably settable in a recording and/or playback apparatus can be rotated together with a turntable included in a disk rotation drive mechanism with the central portion thereof being clamped to the turntable when used in the recording and/or playback apparatus. The optical disk rotated with the central portion thereof being clamped to the turntable is likely to incur the so-called “axial runout”. That is, it deflects axially about its clamped central portion while being rotated.
If the optical disk set in the recording and/or playback apparatus axially runs out while being rotated by the disk rotation drive mechanism, mainly a focus error is likely to occur during recording or reproduction of information.
Therefore, the optical disk removably settable in the recording and/or playback apparatus should desirably be mounted horizontally on the turntable without tilting when it is clamped to the turntable, but it is difficult to place and clamp the optical disk precisely horizontally on the turntable.
Such being the case, the focus servo mechanism provided in the optical disk recording and/or playback apparatus using a removable optical disk as a recording medium is addressed to some axial runout the optical disk set in the apparatus will possibly incur while being rotated by the disk rotation drive mechanism. For DVD (Digital Versatile Disk) for example, the focus servo mechanism of the recording and/or playback apparatus is so designed that even an axial runout of ±300 μm in relation to the plane of disk rotation will not cause any focus error.
These days, it has been proposed to utilize evanescent light in an apparatus that records or reproduces predetermined information to or from an optical recording medium such as an optical disk by irradiating a light beam in order to enable recording or reproduction with a high density beyond the limit of light diffraction.
For utilizing evanescent light to record information to an optical recording medium and/or reproduce information recorded in an optical recording medium, it has been proposed to use an SIL (Solid Immersion Lens) for emission of the evanescent light.
To record or reproduce information to or from an optical recording medium with the use of the evanescent light, a two-group lens which is a combination of an SIL and aspheric lens and of which the numerical aperture (NA) is more than one (1) should be used as an optical system that condenses a light beam for irradiation to the optical recording medium in order to reduce the distance between the light-outgoing surface of the optical system and information recording surface of the optical recording medium to less than a half of the wavelength λ of the light beam incident upon the SIL. For example, in case the wavelength λ of the light beam is 400 nm, the above distance should be less than 200 nm.
For quality recording or reproduction, the distance between the light-outgoing surface of the focusing optical system and information recording surface of the optical recording medium should be kept constant. On this account, there is disclosed in the Japanese Published Unexamined Patent Application No. 2001-76358 (Patent Document 1) a technology in which in an optical head including an SIL, a difference between return light from the optical recording medium is utilized as an error signal to control the position of the focusing optical system in relation to the optical recording medium by driving and controlling the actuator of the optical head correspondingly to the error signal.
For implementing the quality recording and reproduction with the use of evanescent light, it is necessary to have the optical head follow the information recording surface of the recording medium while controlling the distance between the light-outgoing surface of the focusing optical system and information recording surface of the optical recording medium at extremely short intervals of nano meters.
Also, the disk-shaped recording medium itself incurs some radial runout which will prevent the actuator of the optical head from accurately following a beam spot. Therefore, the tracking should be servo-controlled by a tracking servo mechanism.
In the past, in the tracking servo control made in a tracking servo controller and focus servo control made in a focus servo controller, a tracking error signal and focus error signal influenced by disturbances such as axial runout and radial runout have been detected, servo error signals free from the influences of the disturbances have been extracted from the detected tracking and focus error signals and the tracking and focus servo mechanisms have been controlled until the servo error signals become zero.
Also, the servo error signal has been stored in a memory and an error signal stored in the memory has been used to suppress the disturbance components included in the servo error signal. In this case, the control system is a dead-time one, and a strictly proper control system having numerous poles on an imaginary axis will not be able to stabilize any closed-loop system if it is left as it is. On this account, the error signal is passed through a filter before being stored in the memory, not stored as it is into the memory, to stabilize the closed-loop system.
In the past, however, the multiplication factor of the filter intended to stabilize the closed-loop system has been artificially selected by try and error to meet conditions for the stability. Therefore, designing the filter has taken much time, the filter performance has depended upon the designer, and the quality of servo control has not been constant.